Analyzing Shallow Basin Effects in Los Angeles Basin using 3D Simulations and Dense Array Analysis

Voon Hui Lai, Robert W. Graves, Zhongwen Zhan, Chunquan Yu, & Donald V. Helmberger

Published August 15, 2019, SCEC Contribution #9804, 2019 SCEC Annual Meeting Poster #012

Ground motions in the Los Angeles Basin during a potential large earthquake such as a San Andreas rupture are modulated by several factors including earthquake source magnitude and rupture length, path effects into the LA Basin, and local basin effects. In this study, we analyze the direct effect of shallow basin structures on ground motion intensity and duration at long period (up to 2 seconds) in the Los Angeles region through modeling local, small magnitude earthquakes and dense array analysis. Observations show that shallow events produce longer and stronger shaking compared to deeper events. 3D simulations using CVMS4.26 show good waveform fitting at 5 seconds and longer, but fail to predict the long significant shaking and late arrivals at shorter period. The implementation of higher Q in the simulations improves the waveform fitting but still does not fully reproduce the late arrivals. Beam-forming analysis using a dense array within the basin further reveals important factors that control ground motion at long period. The strongest shaking is generated along the great circle path, with increasing slowness, indicating the role of shallow, slow-velocity structures near the surface. Later arrivals are scattered from off-azimuth directions, with potential scatterers such as sharp boundaries offshore. Better imaging of these shallow heterogeneities and sharp boundaries will improve our overall capability in predicting ground motions in future earthquakes.

Key Words
ground motion, basin effect, dense array

Citation
Lai, V., Graves, R. W., Zhan, Z., Yu, C., & Helmberger, D. V. (2019, 08). Analyzing Shallow Basin Effects in Los Angeles Basin using 3D Simulations and Dense Array Analysis. Poster Presentation at 2019 SCEC Annual Meeting.

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Ground Motions